Light-emitting devices having optical semiconductors such as light-emitting diodes (LEDs) are spreading in recent years as substitutes for incandescent lamps or fluorescent lamps. Although LEDs are inferior to conventional illuminators such as fluorescent lamps in luminance per element, the size of each element is up to 1 mm at the most. Therefore, a light-emitting device having an LED mounted thereon is usually produced by disposing two or more LED elements on a substrate.
In general, a method for producing a light-emitting device having an LED includes mounting LED elements on a substrate, followed by encapsulating the elements. As a technique for LED element encapsulation, a method in which the elements are embedded in an organic resin by injection molding or potting is known. However, this technique has drawbacks, for example, that it necessitates the trouble of dropping a given amount of a liquid resin onto the individual elements. Therefore, there is a desire for an encapsulation technique which can be practiced more easily.
JP-A-2005-294733 discloses a sheet for encapsulation which has a multilayer structure composed of resin layers differing in refractive index and which not only is effective in easily conducting resin encapsulation but also produces an effect that the optical semiconductor device obtained can retain a high efficiency of light takeout. JP-A-2006-140362 discloses a sheet for encapsulation which has a multilayer structure composed of resin layers differing in refractive index and a light-diffusing layer interposed between the resin layers from the standpoint of diffusing the directional properties of light.